1. Field of the Invention
The present invention is directed to the direct manufacture of pigmented polyurethane powders and to the powders obtained therefrom.
2. Description of the Prior Art
The demand for thermoplastic resin powders is increasing for such uses as coatings, adhesives, thin films, and foils. The number of applications where these thermoplastic resin powders must be pigmented is also rapidly increasing as many manufacturers replace products and processes which discharge solvent materials into the atmosphere with products containing no solvents.
In general, resin powders have been prepared by grinding already formed resinous materials or by dissolving already formed resin and precipitating it from solution. It has also been shown that solid resinous materials can be produced by coagulation of resin lattices. Also, it is known that polyurethanes can be prepared by the interfacial polymerization of polyisocyanates with compounds containing amine or hydroxyl groups when the reaction is carried out in the presence of an inert liquid medium where at least one of the reactants is insoluble in the medium. The use of special surfactants also allows one to make very fine particles of a uniform size which may be below 100 microns.
The simple addition of dry pigments to these thermoplastic resin powders can cause problems with the practical applications of the pigmented resin powders. Due to the differences in particle size of the resin powder and the pigment particles as well as the differences in densities of the two materials, settling of the pigment can occur during transport and storage of the pigment/resin mixture. This will result in inconsistencies in color tone and intensity of the resulting pigmented polymer when it is used. Often pigmented films formed from resins prepared in this manner have very poor surface appearance. Bubbles often occur in the surface due to the release of a large amount of gases which are normally present on the pigment surfaces. Also, agglomerated pigment particles can cause a rough surface especially when thin films of the pigmented materials are made.
Normally, pigmented thermoplastic resin powders are produced by melting these resinous powders, or thermoplastic resins prepared by other conventional methods, and then blending them with the pigment(s) and finally grinding the pigmented resins into finely divided solids. Because the resinous materials to be ground are thermoplastic and often tough materials, relatively costly and technically impractical procedures involving chilling the resin, for example with liquid nitrogen, have been necessary for effective grinding of the pigmented thermoplastic resins. In addition to the cost of the refrigerant and the grinding equipment, sophisticated collection equipment has been necessary because of the substantial amount of dust which is generated in the grinding process. Also it is difficult to assure that moisture which is condensed onto the product during and after the cooling process is excluded from the finished pigmented thermoplastic powder. Finally, the irregular shapes of these powders can also cause poor flowability which hinders the transfer, handling and application characteristics of these materials.
It is an object of the present invention to manufacture a homogeneous pigmented resin powder directly, i.e., without grinding an already formed thermoplastic pigmented resin into a powdered form.